Liquid nitrogen is used as a nonflammable and portable cold source in various industries. Furthermore, low temperature, solid nitrogen is poorly transfers heat when cooling a solid surface. Additionally, nitrogen cannot be used as a fluid, and hence its usage is limited.
Recently, a high temperature super conductive material has been discovered and practically applied. Consequently, a super conductive system capable of cooling at the same temperature as a liquid nitrogen temperature has emerged. A temperature as low as 65 K is a limit for cooling without solidifying liquid nitrogen, which has a melting point of 63 K. Since an upper limit just before boiling is approximately 75 K, a temperature range capable of cooling with a sensible heat of liquid nitrogen varies by 10 K.
Since the specific heat of liquid nitrogen is 2 kJ/kg, a heat content per unit mass of liquid nitrogen capable of variation of sensible heat is only 20 kJ/kg. Further, as a matter of fact, a property of a super conductive body is usually more stable at a low temperature (near the solidifying temperature of liquid nitrogen) than at a high temperature (near the boiling temperature of liquid nitrogen.)
Thus, since a temperature range capable of cooling utilizing a sensible heat of liquid nitrogen is narrow and its heat content is small, a great deal of liquid nitrogen is necessary. Hence, a large super conductive apparatus becomes necessary. If a cooling temperature rises to a boiling point, for example, a performance of the super conductive body is limited to the temperature thereof. Consequently, using a slurry of solid nitrogen particles and liquid nitrogen, so-called slush nitrogen, that is capable of being utilized together with a latent heat of phase change (25 kJ/kg) , makes it possible to maintain the low temperature near the triple point of nitrogen (63K.) Hence, the aforementioned difficulty can be overcome.
There also is a demand for slush hydrogen as liquid fuel for the space industry. For example, a slush hydrogen producing apparatus is disclosed on Japanese laid-open patent publication No. 6-241647(1994), wherein hydrogen is liquefied with a heat exchanger by utilizing cold liquid helium. A part of the liquid hydrogen is further solidified on a solid surface and cooled by the liquid helium, and the generated solid hydrogen is scraped.
Japanese laid-open patent publication No. 6-281321(1994) discloses a method and an apparatus for producing slush hydrogen by solidifying liquid hydrogen on a solid surface cooled by cold liquid helium in a cryostatic container and scraping the generated solid hydrogen, wherein a great deal of slush hydrogen is produced by releasing the super cooled liquid hydrogen into the cryostatic container.
In Japanese laid-open patent publication No. 8-283001(1996), a method and apparatus for producing slush hydrogen, in which solid hydrogen is generated by blowing hydrogen gas into liquid helium and triple point liquid hydrogen is mixed with the generated solid hydrogen.
In each case, hydrogen is changed by the application of helium. Suppose the art is applied to the production of slush nitrogen. When helium is recycled by condensing it as a cooling agent, a liquefying machine is necessary and the temperature for liquefaction of helium needs to be lower than the temperature for liquefaction of nitrogen or hydrogen. Such a process has the disadvantages of requiring a big apparatus and having a high production cost.
Therefore, the applicant of the present invention proposed methods for producing slush nitrogen on unpublished patent applications JP 2003-065571 and JP 2003-391508. The former is an art in which slush nitrogen is produced by sucking liquid nitrogen in a low temperature vessel using a low temperature refrigerant capable of generating solid nitrogen as a working fluid. Because only helium can be used as a low temperature refrigerant to generate solid nitrogen, a particular means is needed in order to prevent occluding an ejector nozzle with solid nitrogen. As for control of solid nitrogen particle size, though it can be possible in a way by a nozzle diameter or a pressure of fluid, there are too many factors to control in order to obtain stably fine particles having a uniform particle diameter. The latter is an art in which a gas phase in a vessel of liquid nitrogen is depressurized to evaporate nitrogen in a liquid phase so as to lower a temperature finally to the triple point of the nitrogen; solid nitrogen is generated by keeping the triple point and generated solid nitrogen is finely pulverized to obtain slush nitrogen. However, control of a particle diameter is difficult.
Meanwhile, as industrial application of slush nitrogen has advanced, the flow property of a fluid has become essential. More specifically, it is required that solid nitrogen particles in the slush nitrogen are finely pulverized and a particle size is made uniform so as to exert a good flow property of little pressure loss. Thus, when a long and big body is cooled in flowing slush nitrogen, or slush nitrogen is transferred for a long, distance, cooling or transferring can be realized with high performance and high efficiency.